How does a meniscus get torn?

Many ways. Twisting injuries typically cause a meniscus tear. However, as we age, degenerative tears can also occur. Surgery is usually only indicated for acute injuries where the exam, symptoms, and MRI suggest that the meniscus tear is causing pain, or the torn tissue is blocking full range of motion.
Usually by twisting. The menisci are attached to the top of the tibia, shin bone, and cradle the end of the femur or thigh bone. These are the two longest bones in the body. As they move the meniscus should glide with the smooth surfaces of these two bones. If it gets caught between them a relatively minor seeming force can apply enough shear force to tear the meniscus.